This invention relates to a method of controlling the switching of a semiconductor switching device, including at least one PNPN junction so as to reduce the turn-off time thereof and also to increase both the OFF-state voltage and a dv/dt capability.
A description will now be given, by way of example, of the switching of silicon controlled rectifiers (SCR's) most typical among PNPN semiconductor switching devices.
In order to switch an SCR from its ON state to its OFF state, the usual practice is to forcedly reverse the voltage across the main electrodes or the anode and cathode electrodes thereof by means of an external circuit and maintain the resulting reversely biased state for a predetermined time interval. The minimum time interval between the instant when the principal current has decreased to zero and the instant when the SCR is capable of supporting a reapplied positive anode voltage without turning on is called the turn-off time. In SCR's, the turn-off time can be variously changed in accordance with the operating conditions such as the waveforms of current and voltage, and the temperature imparted to the SCR's. Among them it is to be noted that if a signal is applied across the gate and cathode electrodes of SCR's, their turn-off time changes. This signal is called a "reverse gate bias pulse" hereinafter.
There is a well-known phenomenon that when a reverse gate pulse is applied across the gate and cathode electrodes of SCR's having the main electrodes reversely biased with respect to each other, the turn-off time thereof is decreased. The mechanism by which this phenomenon occurs should readily be understood from the construction of SCR's and need not be described in detail herein.
There has been often demanded for various applications SCR's having shorter turn-off times and yet which are still able to control high voltage and large currents. To this end, it has usually been the practice to dope the silicon substrate of the SCR's with a heavy metal such as gold acting as a lifetime killer, thereby to reduce the turn-off time. However, this measure has been attended with the undesirable phenomenon that the shorter the turn-off time, the more the other important characteristics, such as the ON-state voltage, OFF-state voltage, etc. will deteriorate. Therefore, SCR's for switching high electric voltages and large currents at high speeds have had an upper limit beyond which their performance cannot be improved. Under these circumstances, it is quite proper to introduce the effect of reversely biasing the gate.
It will be found that, upon the application of the reverse gate bias pulse to high power, high speed SCR's, the turn-over time is not decreased very much. This is mainly attributable to the shorted emitter structure. In order to decrease the turn-off time, the shorted emitter structure may be removed from the SCR's. However, if the shorted emitter structure is removed from the cathode region of SCR's having a relatively large area, such as in high power SCR's, then the injection efficiency at the associated junction is greatly decreased to greatly lower the dv/dt capability, where dv/dt is a rate of rise of the OFF-state voltage. Thus, upon applying the forward voltage across such SCR's, the resulting multiplied current is increased, resulting in a decrease in OFF-state voltage and the like.